High power radar system with failsafe receiver protection



H. GOLDIE Aug. 6, 1968 2 Sheets-Sheet 1 Filed Feb. 13, 1967 {I 3W $8.5 93.. o I. 3% n Wlo a $255 A b NQ C K 591 2 29.52%: mm mm "624316 M N 225.2 BL tz: L +m ll I I I'lllu I I. 20533. e 1 1i Iv 3 Iv mm x5352 wzEmE M331 3% INVENTOR Harry Goldie 2% ATTORNEY GvTmissEs w XM v7 Aug. 6, 1968 GOLDIE 3,396,388

HIGH POWER RADAR SYSTEM WITH FAIL-SAFE RECEIVER PROTECTION Filed Feb. 13, 1967 2 Sheets-Sheet 2 TRIGGER PULSE A k) I I I ARc CURRENT PULSE B n) l I I I MAGNETRoN VOLTAGE PULSE I C m) I I I I I I BRIEF I,I: I'RF PULSEWIDTH RF HIGH POWER PULSE DELAY I D (ANTENNA OUTPUT) u I I l ANTENNA REFLECTED PULSE I DB FROM CURVE D E (Kl LOWATTS) I I I NO SPIKE LEAKAGE\I -70 DB FROM cuRvE E F RECEIVER P oTEcToR LEAKAGE i "1 70 DB ATTENUATION OF Tws RECEIVER PROTECTOR 3 DB ISOLAT'ON Q l INITIATES AT To United States Patent i l 3,396,388 HIGH POWER RADAR SYSTEM WITH FAIL- SAFE RECEIVER PROTECTION Harry Goldie, Randallstown, Md., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 13, 1967, Ser. No. 615,476 7 Claims. (Cl. 343-) ABSTRACT OF THE DISCLOSURE Fail-safe receiver protection in a high power radar system is provided by a thyratron wave guide switch, hereinafter referred to as TWS, inserted between the circulator and the receiver. Since a TWS is a hydrogen thyratron device in which a section of rectangular wave guide is used as a control grid the wave guide section constituting the grid is a part of the wave guide transmission line. This line is located between the circulator and the receiver while the cathode-anode circuit of the TWS constitutes the switch in the primary circuit of the magnetron pulse transformer to discharge the pulse forming network grid. Thus, in effect, an electrical interlock is provided which insures that the receiver protection is initiated before the magnetron oscillation burst can start. The TWS is operated in grounded-grid circuit configuration.

In copending application Serial No. 615,507 filed February 13, 1967, in the names of Harry Goldie, L. F. Cooper and H. W. Cooper, and entitled Ground-Grid Cathode Driven Triggered Waveguide Switch, there is disclosed and claimed the grounded-grid circuit configuration of a thyratr'on wave guide switch, hereinafter referred to as TWS. The advantage of the grounded-grid circuit configuration is that the grid of the TWS can be operated at common ground potential with the wave guide transmission line, thus eliminating the insulation and capacitance problems.

In copending patent application Serial No. 383,639, filed July 20, 1964, in the name of Harry Goldie, now Patent No. 3,332,003, a wave guide switch of the type contemplated by this invention is described. This device is basically a hydrogen thyratron electron discharge device in which a sectional rectangular microwave guide serves as the grid. The sectional microwave guide is sealed into the envelope that encloses the cathode and anode and certain portions of the wave guide section in the gaseous atmosphere. The pressure windows extend across the opening in the wave guide at spaced points to complete the sealed envelope. The side walls of the wave guide section are provided with apertures to permit the electrons to flow transversely across the section of the wave guide between the cathode and anode. The stream of electrons carry the electric current when the tube is fired to create a dense plasma in the wave guide which forms an electromagnetic wave barrier, thus producing the attenuation to constitute the switching action and receiver protection.

The present invention utilizes a TWS, preferably in the grounded-grid circuit configuration, although this is not essential, to provide an electrical interlock between the high power transmitter of the radar system and the receiver to protect the latter. It is believed that the figure 3,396,388 Patented Aug. 6, 1968 of the drawing will be substantially self-explanatory to those skilled in the art. From the drawing, it is seen that a section of wave guide, serving as the control grid for the TWS, is inserted in the wave guide transmission line between the circulator and the receiver. The cathodeanode circuit of the TWS serves as the triggered electrical switch in the primary circuit of the pulsing transformer for the magnetron generator and the plasma generated in the wave guide section grid constitutes an RF barrier in the Wave guide section grid. Since there is a slight time delay in the initiation of the operation of the magnetron after the power is applied to it, it is impossible for an oscillation burst from the magnetron to reach the receiver.

It is apparent from the above that it is the purpose of this invention to provide a novel and improved fail-safe protection for a high power radar receiver.

Other and further objects will be apparent from the following description when considered in connection with the accompanying drawing in which:

FIGURE 1 is a schematic circuit diagram of a radar transmitter and receiver incorporating the present invention; and

FIG. 2 is a graphical illustration of the sequence of events in the operation of the present invention.

Heretofore, in radar systems the duplexer, used to isolate the receiver from the transmitter burst, was slaved to the burst instead of being separately synchronized with some provision for initiating the receiver protection before the initiation of the transmission burst. As a result, the isolation of the duplexer, not being an independently actuated function, is somewhat delayed allowing the leading edge of the transmission burst to reach the receiver unattenuated, that is, a certain amount of the spike leakage energy from the transmitter burst leaks through to the receiver. This often caused poor, and often unreliable, operation and led to great efforts being expended on ignitor designs which would reduce the amplitude of the spike leakage.

It is a purpose of the present invention to provide a novel technique whereby a necessary condition precedent for the magnetron pulse to exist is that the receiver protection exist first. This provides a fail-safe mode of operation; that is, the transmitter cannot radiate before the receiver is completely isolated. If the receiver isolation fails it would be impossible to transmit a high RF power burst. The fact that the receiver protection isolation .precedes the transmitter burst completely eliminates the usual spike leakage problem.

Referring to FIG. 1 of the drawing, a schematic circuit diagram of the present invention includes a magnetron generator 10, a conventional circulator 11 having an arm 12 which is connected to a wave guide transmission line feeding an antenna (not shown), an impedance matching arm 13 and a fourth arm 16 which is part of a Wave guide transmission line 17 connected to a radar receiver RCVR. A triggered wave guide switch 18, which conventionally uses a section of wave guide 19 as the grid electrode, is interposed and connected between arm 16 and the remainder of the transmission line 17 to the receiver. The wave guide switch 18 includes an envelope 21, which may be made of glass or metal, sealed gas-tight to wave guide section 19. The wave guide section 19 is provided with pressure windows 22 and 23 which form a .3 gas-tight enclosure for that portion of the wave guide section sealed to the envelope 21. The envelope is evacuated and filled with pure hydrogen in accordance wit-h the conventional practice. The side walls of the wave guide section 19 are provided with a plurality of apertures 24 and 26 to permit the flow of electrons from the cathode 27 to the anode 28. The wave guide section 19 serves the dual function as a section of a wave guide transmission line while at the same time the perforated sidewalls serve as the control electrode for the thyratron switch 18. In the non-conducting condition microwave energy can be propagated through the wave guide section 19 from the arm 16 of the circulator to the remainder of the wave guide transmission line 17 with negligible power loss. With appropriate negative bias on the cathode 27 an electric arc will form between the cathode 27 and the anode 28, thus generating a dense plasma, indicated at 31 in the wave guide section 19, which serves as an RF barrier to block the propagation of microwave energy between the arm 16 and the transmission line 17.

To provide the fail-safe protection for the receiver a suitable soure of direct current potential represented by the terminal 32 is connected through an isolation unit 33 to two parallel circuits, one of which includes the anodecathode circuit of the thyratron 18 and the other of which includes a pulse forming network 34 and the and the primary Winding 36 of the pulse-forming transformer 37 and common ground G. The pulse transformer 37 has a secondary 38 which is connected across the cathode 39 and the anode 41 of the magnetron to provide the energization for the operation of the latte-r.

A trigger pulse generator 44 is adapted to supply a negative pulse e from the terminal 46 to the cathode 27 of the thyratron 18. In accordance with the circuit configuration described in the first aforementioned copending patent application, the cathode circuit of thyratron 18 includes a diode D having connected in parallel therewith a resistor R and having its anode 46 connected ohmically to the cathode 27 of the thyratron 18. The resistance value of the resistor R is several times that of the internal resistance of the trigger pulse generator 44 in order not to heavily load the output of the trigger generator and the diode D is in open circuit condition for negative pulses. With this resistor-diode combination in the cathode circuit of the thyratron 18, when the trigger pulse generator delivers a negative pulse it sees a resistive load when the thyratron is unfired. When the negative pulse from the trigger pulse generator 44 biases the cathode 27 to conduction the diode D conducts, which effectively short circuits the resistor R The diode then passes the full arc current, which may be typically in the neighborhood of 10 to 50 amperes and of 0.5 to 2.5 microseconds .duration, and changes from the non-conduction to the conduction state in less than 0.1 microsecond. As soon as the TWS commutates its discharges the pulse forming network 34 and the heavy arc current fiOWs along the dotted path 47 which includes the cathodeanode circuit of the thyratron 18 and the plasma 31. The plasma 31 constitutes an RF barrier and approximately 70 db attenuation to any microwave energy which might try to pass through the wave guide transmission line from the arm 16 to the transmission line 17. It will be apparent that the discharge of the pulse-forming network 34 through the primary 36 of the pulse transformer 37 will apply a voltage pulse across the cathode-anode circuit of the magnetron 10 producing a burst of radio frequency to be delivered to the input arm of the circulator 11. The burst of RF oscillations from the magnetron will commence after a brief interval after the voltage pulse from transformer 37 is applied across the magnetron. However, in the meantime the RF barrier in the plasma 31 has formed before any RF energy reaches the TWS 18.

From the above it will be apparent that the salient feature of the present invention produces a fail-safe mode of operation such that the magnetron 10 can oscillate only when current flows through the plasma 31 between the cathode 27 and the anode 28 of the thyratron 18. Since the plasma 31 serves as a barrier to the radio frequency, protection of the receiver RCVR is assured. Furthermore, since the current in the cathode-anode circuit of the magnetron must flow prior to invention of the magnetron oscillations, the high density plasma 31 will have time to form and will be in a high state of isolation prior to the burst of microwave energy. This means that the leading portion of the transmitter burst is attenuated the same amount as the remainder of the burst.

The operation literally described above is graphically illustrated in FIG. 2 where curve (A) shows the negative output pulse c from the trigger pulse generator 44. At (B) it is indicated that the current 1' in the cathode-anode circuit of the thyratron 18 is initiated sometime later at t Then after a very brief delay, after t the voltage pulse from the transformer 37 is supplied to the magnetron 10 at t as indicated at (C) in FIG. 2. Almost simultaneously therewith at z; the radio frequency pulse from the magnetron is initiated and continues for the duration of the voltage pulse applied to the magnetron as indicated at (D) in FIG. 2. The antenna reflected pulse in kilowatts is indicated at (E) while the level of the RF energy which reaches the receiver RCVR is indicated at (F). It is to be noted that the transmitter leakage energy that reaches the receiver RCVR is db down from the amplitude of the antenna reflected pulse in (E). The curve (G) in FIG. 2, on the other hand, indicates that the RF barrier created by the plasma 31 in the thyratron 18 has provided the isolation and protection for the receiver RCVR at time t which is the same instant that the current i begins in the cathode-anode circuit of the thyratron. It also will be noted that the isolation continues for a brief time beyond the duration of the RF pulse from the magnetron 10.

I claim as my invention:

1. In a radar system including a radio frequency generator, a radar receiver, an antenna for radiating electromagnetic wave energy supplied by said generator, a circulator, transmission line means connected between said generator, said circulator, said antennna and said receiver for directing the major portion of the radio frequency energy from said generator to said antenna and for directing received radio frequency energy from said antenna to said receiver, an electronic waveguide switch having a cathode, an anode and a control electrode in the portion of said transmission line means between said circulator and said receiver, circuit means for energizing said generator including the electron stream between the anode and cathode of said switch, whereby the electron flow in said switch creates an RF plasma barrier in the portion of said transmission line means between said circulator and said receiver before said generator can supply microwave energy.

2. The combination as set forth in claim 1, in which said switch is a triggerable electron discharge device.

3. The combination as set forth in claim 1, in which the portion of said transmission line means between said circulator and said receiver is a waveguide.

4. The combination as set forth in claim 3, in which the electron stream of said switch is transverse to said waveguide.

5. The combination as set forth in claim 3 in which said control electrode is in the form of a waveguide section coupled to and constituting a continuation of said transmission line means, said waveguide section having perforated side walls to permit electron flow while serving as said control electrode of said switch.

6. The combination as set forth in claim 5, and including a pulse-forming transformer having a secondary connection to energize said generator and a primary connected in series with the cathode-anode circuit of said switch device, a source of direct current potential con- 3,396,388 5 6 nected to said anode and said pulse-forming network, and References Cited means for biasing the control electrode of said switch to UNITED STATES PATENTS conduction condition to discharge said pulse-forming network. 3,023,380 2/1962 Hill 333 -13 7, The combination as set forth in claim 1, in Which 5 3,085,239 f/1963 Hoover 3435 aid circuit means for energizing said generator includes 33302698 3/1967 Baldock 3435 21 pulse-forming network which is discharged through said electron stream to close the circuit through said energiz- RODNEY BENNETT Examiner ing circuit means. C. L. WHITHAM, Assistant Examiner. 

